1. Field of the Invention
The present invention relates to pulse width modulation (PWM) controllers. More particularly, the present invention relates to a three-terminal, low-voltage, pulse width modulation (PWM) controller integrated circuit (IC) for providing source control of a switching element within an electrical circuit such as an off-line switching power supply, DC-to-DC boost converter, or motor controller.
2. Introduction to the Invention
It is very desirable to minimize the cost, size and power dissipation of a low-cost off-line switching power supply for low power applications, such as recharging cells and batteries used in portable consumer appliances, such as entertainment units, personal digital assistants, and cell phones, for example. One method to achieve these desirable goals is to use one or several integrated circuits in standard packages, whether dual-in-line (DIP) or three-pin to realize a pulse width modulation (PWM) controller to control power supply switched duty cycle.
PWM controllers are typically designed to regulate the output voltage of a switching power supply. A PWM switched power supply requires a variable pulse width that is controlled by an error signal derived by comparing actual output voltage to a precise reference voltage. The pulse width of the switching interval must also be constrained to be within a minimum and maximum duration. These constraints are imposed for correct PWM power supply or motor driver operation.
One example of a conventional three-pin PWM controller integrated circuit (IC) is found in U.S. Pat. No. 5,313,381. While this prior approach appears to have been satisfactory in its time, it has several drawbacks. First, it employs both bipolar and MOS technologies, and therefore has added fabrication process complexities. Second, it includes an on-board high voltage power switching field effect transistor (FET). The high voltage switching FET takes up approximately half of the chip area, and the area remaining for other circuits becomes limited. Because the high voltage switching FET is on-chip, special steps must be followed in chip design and fabrication to accommodate the high voltage swings present at the chip during switching circuit operation. Also, the high voltage switching FET requires a relatively large IC package capable to meet the creepage requirements as well as conducting heat to an external heat sink, such as a TO-220 three-terminal package having a heat sink tab, and has a thermal dissipation limit which ultimately restricts the practical amount of power that can be switched in a switching power supply using this prior approach irrespective of external heat dissipation arrangements. Furthermore, since no voltage is available at the secondary side of a switching power supply at start up, a special low voltage tap within the high voltage FET structure must be provided and used in this prior approach in order to obtain primary side start-up voltage.
Thus, a hitherto unsolved need has remained for a low cost, low voltage PWM controller IC which may be packaged in a variety of available, low cost IC packages depending on application in a manner overcoming limitations and drawbacks of the prior art approaches.
A general object of the present invention is to provide a three-terminal low-cost, low-voltage PWM controller IC architecture overcoming limitations and drawbacks of the prior art.
Another object of the present invention is to provide a low-voltage PWM controller IC for providing source control of an external current-regulating circuit element, such as a high-voltage switching transistor.
Another object of the present invention is to provide a low-voltage PWM controller IC which does not require high voltage insulation and conduction of excessive heat to the external ambient and which may therefore be packaged in a variety of low cost plastic packages, including three-terminal packages.
Another object of the present invention is to provide switching power supply primary side topologies effectively configured to use the low-voltage PWM controller IC of the present invention.
Another object of the present invention is to provide an on-chip start-up circuit for providing initial start-up power to start the PWM controller of the present invention, without requiring an additional high voltage device.
A further object of the present invention is to provide a PWM controller which may be implemented as an integrated circuit employing a low voltage IC fabrication process.
One other object of the present invention is to provide a three-terminal lowv-oltage PWM controller IC which responds to a plurality of error parameters in developing width modulation control pulses, such as output voltage feedback as well as internal switching current flow controlling the external electrical circuit.
In accordance with principles of the present invention, a three-terminal low-voltage pulse width modulation (PWM) controller is embodied in a unitary integrated circuit (IC). The circuit includes a first terminal providing an input node for receiving operating bias current supply and a feedback control signal related to an output parameter of an electrical circuit controlled by the digital PWM controller; a second terminal providing an output node connected to a digital output switch providing digital width-modulated control pulses at a control pulse rate to control duty cycle of the electrical circuit, the digital width-modulated control pulses being width-modulated in relation to the feedback control value; a third terminal providing a ground connection; clocked pulse width modulation circuitry responsive to current flow between the second terminal and the third terminal and the feedback control value for controlling the digital output switch at the control pulse rate; and, feedback signal separation circuitry for separating the feedback control signal from the operating bias current supply, most preferably a current sensing resistor in series with the digital output switch and the ground terminal for sensing current flow between the second terminal and the third terminal as a voltage.
As one aspect of the present invention, the low-voltage PWM controller IC includes a start-up circuit for initially connecting the second terminal to the first terminal to permit voltage initially present at the second terminal to charge an external storage capacitor connected to the first terminal, until the operating bias current supply is present at the first terminal. The start-up circuit most preferably includes a first transistor and a diode in a conduction path from the second terminal to the first terminal, a second transistor for controlling a gate electrode of the first transistor, the second transistor being controlled by a logic circuit responsive to presence of the operating bias current supply at the first terminal. The logic circuit most preferably includes a high voltage comparator for comparing voltage level at the first terminal with a reference high voltage level, and a latch reset by the high voltage comparator, the latch directly controlling a gate electrode of the second transistor. The logic circuit may also include a low voltage comparator for comparing voltage level at the first terminal with a reference low voltage level, so that if a low voltage condition is sensed to be present at the first terminal, the latch will become set by the low voltage comparator in order to turn on the first transistor and reestablish a conduction path from the second terminal to the first terminal.
As another aspect of the present invention, the three-terminal low-voltage PWM controller is encapsulated in a three lead TO-92 miniature plastic package for through-hole installation into a circuit board of the electrical circuit, a first lead corresponding to the first terminal, a second lead corresponding to the second terminal, and a third lead corresponding to the third terminal.
As one more aspect of the present invention, the electrical circuit is a switching power supply having a switching N-channel FET configured in a common-gate mode. In this arrangement, the second terminal connects directly to a source electrode of the switching FET.
In one more aspect of the present invention, the electrical circuit is a DC-to-DC switching boost converter and the second terminal is connected to shunt an energy-storing inductor to ground.